What I’ve done recently is grab one of the bluetooth FM transmitters for cars, that let you connect your phone and it broadcasts that onto a FM radio station.
They actually reach a lot farther than you think. Now any FM radio in your house will be perfectly in sync, and as many as you want. This has a limit depending on how thick your walls are but they’re cheap as hell and work well.
Mfw we finally create Roko’s basilisk